Polymeric composition for friction elements

ABSTRACT

The polymeric composition for friction elements comprises a co-polymer between (I) a resin containing phenolic groups and a reticulation agent, and (II) an organopolysiloxane resin containing terminal silanol groups. A part at least of the phenolic groups is bound to the terminal silanol groups. A process of the preparation of the above polymeric composition may comprise the following steps: a) mixing (I) a resin containing the phenolic groups and the reticulation agent, (II) containing the terminal silanol groups, and (III) an epoxy resin or the epoxidisesd organopolysiloxane; b) curing the mixture for a period of time sufficient to complete substantially the reaction between the phenolic groups and the terminal silanol groups, c) post-heating the product obtained under b).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is concerned with a polymeric composition for frictionelements having remarkable properties with regard to temperature andcontact with water. Although the invention will be described in moredetails with relation to brake pad or brake linings, it should beunderstood that it may be used in any application in which frictionproperties have to remain stable with increasing temperature and underwet conditions, such as brakes and clutches for vehicles and machinetools. The brake pad is one example in which heat and water are of aprime importance due to a possible overheating if the braking action isapplied for an extended period of time, during which moreover water maycome in contact with the pads.

2. Description of the Prior Art

Preparations or compositions for friction elements for use in brake padsand other applications are known. One example is a mixture in which aphenolic resin and an organopolysiloxane or silicone resin are mixedwith a crosslinking agent as described for instance in EP-0 456 490 andJP-63-251 452.

In the former document a catalyst is used during the preparation, whichconsists of an organic compound conventionally used in the manufacturingof silicone rubber, such as stannous oleate and tin naphtenate.

U.S. Pat. No. 4,657,951 is citing other organic metallic salts which canbe employed as catalysts in the transesterification between a phenolicresin and an organopolysiloxane. Tetrabutyl titanate, tetra isopropyltitanate and butyl polytitanate are mentioned among others. Zirconatesare also known as catalysts for transesterification.

However, according to IR analysis, this mixture appears to be basicallya simple mixture of the original phenolic resin and the product of thehomoreaction between the silicone resin and itself. This means inparticular that the reaction involved do not lead to specificinteractions of the phenolic hydroxy groups with the silicone, most ofthe phenolic groups remaining as such, i.e. as free phenolic groups.Hydrophilic properties are therefore retained together with a relativelyhigh capacity of water absorption, which in turn is affecting stronglythe friction characteristics of the product.

In our co-pending unpublished European patent application no 98420024,6,an improved polymeric composition for friction elements is disclosed,which comprises a co-polymer between (I) a resin containing phenolicgroups and (II) an organopolysiloxane resin or silicone containingterminal silanol groups, a part of the phenolic groups being bound tothe terminal silanol groups. Preferably, the reaction between (I) and(II) is made in the presence of an epoxy resin or an epoxidisedorganopolysiloxane.

BRIEF SUMMARY OF THE INVENTION

As in the above document, the object of the invention is also to makethe reaction between a phenolic resin and an organopolysiloxane orsilicone resin follow a different way, resulting in a actual co-reactionor condensation between the phenolic groups and the silanols groups ofthe silicone in Si—O—C and C—O—C bonds. A part at least of the freephenolic groups of the starting phenolic are consumed in such bonds andwill not longer be available for water absorption. The reaction productwill loose its hydrophilic properties and the water which may come incontact with said product will not be absorbed, yielding a compositionwith improved friction properties even under wet conditions.

DETAILED DESCRIPTION OF THE INVENTION

An object of the invention is products which exhibit still improvedproperties over the products just mentioned above in the unpublishedEuropean Patent Application.

Another object of the invention is to prepare a composition withsuperior heat resistance.

Another object of the invention is to prepare a composition withimproved wet conditions performance.

In other words, the invention relates to a polymeric composition forfriction elements, comprising a co-polymer between (I) a resincontaining phenolic groups, (II) an organopolysiloxane resin or siliconecontaining terminal silanol groups and (III) a reticulation agent, apart at least of the phenolic groups being bound to the terminal silanolgroups, wherein tris neoalkalanate titanate is used as a catalyst forthe co-polymerisation reaction.

Preferably, the resin containing phenolic groups is from 50 to 80% andthe organopolysiloxane resin containing terminal silanol groups is from8 to 25% by weight of the total starting mixture.

The starting resin comprising phenolic groups may also comprise terminalnon aromatic alcoholic groups, a part at least of the terminal nonaromatic alcoholic groups being also bound to the terminal silanolgroups.

The reticulation agent may be an amine such as an hexamine, preferablyto the extend of 8 to 12% by weight.

In one embodiment, the resin containing phenolic groups is of thegeneral formula (A) and may include moieties of a general formula (A′).

A typical convenient resin is for instance a phenyl aralkyl resin inwhich R₁ is —CH₂—φ— having a basic structure of alternated moietiesA/A′. A variety of such resins are sold under the generic name of ®Xylokby Mitsui Toatsu Chemical, Japan.

The other compound, namely an organopolysiloxane resin containingterminal silanol groups may be for instance a diphenyl silicone or anhydroxy phenyl alkyl silicone resin or methyphenylsiloxane.

Among tris neoalkalanate titanates, those neoalkalanates having between2 and 16 C atoms on each chain, saturated or unsaturated, unbranched orbranched, are preferred. These titanates may contain hetero-atoms atomsor heterocyclic moieties.

Examples of convenient catalysts within the above definition are:

neopentyl(diallyl)oxy tridecanoyl titanate,

neopentyl(diallyl)oxy tri(didecyl)benzenesulfonyl titanate,

neopentyl(diallyl)oxy tri(dioctyl)phosphato titanate,

neopentyl(diallyl)oxy tri(dioctyl)pyrophosphate titanate,

their adducts with acrylate and methacrylate functional amines,

neopentyl(diallyl)oxy tri(N-ethylenediamino)ethyl titanate,

neopentyl(diallyl)oxy tri(m-amino)phenyl titanate,

neopentyl(diallyl)oxy trihydroxy caproyl titanate,

all sold by Kenrich Petrochemical under the general name of ®Lica. Apreferred catalyst is neopentyl(diallyl)oxy trineodecanonyl titanate.

The invention relates as well to a process for the preparation of thepolymeric composition, comprising the following steps:

a) mixing and kneading together (I) a resin containing the phenolicgroups with (II) a resin containing the terminal silanol groups, in thepresence of a tris neoalkalanate titanate as a catalyst, for a period oftime sufficient to substantially complete the reaction between thephenolic groups and the terminal silanol groups, to yield a paste,

b) cooling down the paste until obtaining a solid,

c) grinding the solid into a powder,

d) adding the reticulation agent to said powder,

e) curing the above mixture,

f) post-curing the product obtained under e).

In the above processes, the catalyst tris neoalkalanate titanate ispreferably used in quantities comprised between 1 and 2% by weight ofthe total mixture.

If the co-polymerisation reaction is conducted in solution, then thecatalyst may be present in higher quantities, preferably between 1 and18% by weight of the total mixture.

Due to the high efficiency of the reaction in condensing the phenolicgroups with the terminal silanol groups, such a reaction may bedescribed as “end capping”.

The silicone resin is present in the starting mixture from 8 to 25% byweight, preferably around 10 to 20%. Si 6-2230 (a diphenylsilicone) of®Dow Corning Silicone, Z-6018 (an other diphenyl silicone) of ®DowComing or PDS 9931 (an other diphenyl silicone) of ®Gelest Inc. areexamples of such silicone resins.

To make easier the blending of the starting resins, said resins arepreferably in a form of powder with a particle size distribution of notmore than 400 μm, preferably below 300 μm for a compound such as ®Xylokcited above, and 200 μm for silicone.

The mixing and kneading under step a) of the resins and the catalsyt,said catalyst having the consistency of honey, is preferably conductedin a vessel at a temperature of 80 to 135° C. under a nitrogen pressureof 1 to 2 atm. This is easily carried out in a kneading machine with arotation speed between 30 and 80 rpm.

The paste obtained above is then cooled or chilled down, for instancesimply by leaving it at room temperature for a sufficient period oftime. When solid, it is ground to yield a powder with a particle sizedistribution advantageously between 10 and 200 μm.

The hexamine is then added and throughoutly mixed to the powder, and themixture is cured at a temperature between 50 to 200° C., for instancearound 170-180° C., under a pressure of 250 to 320 atm.

In general, the curing step is divided in a number of cycles (5 to 20cycles for instance) permitting the degassing of the reaction mixture.

As to the post-curing, the haeting temperature is advantageously from190 to 240° C.

The various objects and advantages of the invention will become apparentwith regard to the following non limitative examples.

EXAMPLES 1 TO 3

From a starting blend containing a silicone resin of different typesaccording to the examples, a ®Xylok free of reticulation agent(available by Mitsui Chemicals, Japan), and 1,4% ofneopentyl(diallyl)oxy trineodecanonyl titanate (®Lica-01 of KenrichPetrochemicals), samples of the following table 1 were fabricated usingthe parameters given above and in the following additional conditions:

10 degassing cycles with a pressure of 300 atm (6 seconds on, 4 secondsoff) at 175° C.

Post-curing during 10 minutes at 200° C.

TABLE 1 Example Compositions 1 Si-62230 24.25% Xylok 74.35% Lica-01 1.4%2 Z-6018 19.5% Xylok 79.1% Lica-01 1.4% 3 PDS 9931 20.0% Xylok 78.6%Lica-01 1.4%

Specimens of theses formulations were used as a substrate or binder tobe processed into a “simplified” brake pad sample with a minimum numberof ingredients, to form specific formulations for water tests.“Simplified” brake pad is intended to designate a model or simulationprobe for tests traditionally made by using a simplified mix of 4components only, whereas an actual brake pad for vehicles is a morecomplex mixture of over 15 ingredients.

The water absorption was tested using a method, where a 10 μl water dropis deposited at the surface of the sample and the time for absorption isrecorded. On a ®Teflon surface, which was used as a reference, a 10 μldrop disappeared in 60 minutes.

The droplet on a pad surface having a composition corresponding toexample 1 disappeared in 60 minutes when said pad has not been heated,and in 57 minutes when the pad has been submitted to a heating cycle at350° C. during 1 hr. These values indicate no water affinity, very nearto that of ®Teflon, and an excellent resistance to heating andoverheating.

COMPARATIVE EXAMPLE

By way of comparison, a comparative formulation corresponding toexamples 1,2 and 3 was made by the same procedure, but without using thesilicone resin and the catalyst, in order to demonstrate the effect ofthe “end capping”. All formulations of examples 1 to 3 and of thecomparative example were submitted to a heat treatment simulatingheating and over-heating due to a braking action. A temperature of 350°C. was used for 1 hr and 2 hr receptively. Results of the above“droples” tests, expressed in minutes, are given below in Table 2:

TABLE 2 comparative 1 2 3 formulation non heated 66 64 60 37 heated 350°C./1 h 57 54 45 12 heated 350° C./2 hr 16 14 17 8

Conclusions are clear. Formulations of examples 1 to 3 are far superiorto the comparative formulation prepared without the tris neoalkalanatetitanate catalyst. Times to absorb water are at least twice as much. Thepeak of almost 5 times (57, resp. 54, resp. 48 vs 12) for a heattreatment of 1 hr is of a prime importance, since it is a more realistictime for an excessive braking action (descending under full loadconditions).

Another comparison between Example 1 and the same comparativeformulation is given below in Table 3. This time, test samples areactual brake pads using all the traditional ingredients.

TABLE 3 comparative example 1 formulation Water absorption non heated 4044 in minutes heated 350° C./1 hr 35 28 heated 350° C./2 hr 17 9 Bendingstrength non heated 3.87 3.99 in kgf/mm² heated 350° C./1 hr 2.15 1.41heated 350° C./2 hr 1.80 1.12

Again resistance to water and mechanical properties, although comparablebefore heat treatment, are far better after heat treatment.

EXAMPLES 4 AND 5

The following products of examples 4 and 5 were prepared in solution asfollows:

Ex. 4: {fraction (3/7)} of ®Xylok, {fraction (3/7)} of silicone resin Si62230 and {fraction (1/7)} of neopentyl(diallyl)oxy tridecanoyl titanate(®Lica-01) as a catalyst by weight were mixed for 2 hr at 155-160° C.

Ex. 5: {fraction (3/7)} ®Xylok, {fraction (3/7)} of silicone resinZ-6018 and {fraction (1/7)} of neopentyl(dialtyl)oxy tridecanoyltitanate (®Lica-01) as a catalyst by weight were mixed for 2 hr at155-160° C.

COMPARATIVE EXAMPLES

For comparison purposes, the following products were prepared insolution in DMF, as described above:

Comp. Ex 4: ®Xylok ({fraction (3/7)} by weight), silicone resin Si 62230({fraction (3/7)} by weight) and tetra isopropyl titanate ({fraction(1/7)} by weight) of the art (as mentioned above) as a catalyst byweight were mixed during 2 hr at 155-160° C. Tetra isopropyl titanatewas ®Tysor R TPT made by ®DuPont Chemicals.

Comp. Ex. 5: ®Xylok ({fraction (3/7)} by weight), silicone resin Z-6018({fraction (3/7)} by weight) and tetra isopropyl titanate ({fraction(1/7)} by weight) of the art as a catalyst by weight were mixed during 2hr at 155-160° C. Tetra isopropyl titanate was also ®Tysor R TPT made by®DuPont Chemicals.

All above products of examples 4 and 5, and of the comparative examples,were examined by ultra violet spectroscopy after addition of a smallamount of KOH in order to ionise the phenolic groups, which showabsorption at 280 and 275 nm. Results are given below, in Table 4.

TABLE 4 examples 280 nm 275 nm 4 no peak no peak 5 no peak no peakComparative 4 1.84 1.94 Comparative 5 2.24 2.27

As seen from Table 4, the products made with the tris neo alkalanatetitanate catalyst did not show any peak at both wavelengths responsiblefor phenolic OH absortion. It should be noted that the catalysts wereused in amounts in the range of 10 to 15 times of what is neededaccording to the invention. Even with such concentrations of catalyst inthe starting mixtures, the product obtained by using tetra isopropyltitanate as a catalyst did exhibit absorption of phenolic groups.

OTHER COMPARATIVE EXAMPLES

A solution of 7.5 g of ®Xylok and 7.5 g of silicone resin and nocatalyst, respectively 2 g of nonaloxy zirconate as a catalyst, wasmixed for two and four hours.

The product was examined by ultra violet spectrophotometry after theaddition of a small amount of KOH in order to ionise the phenolic groups(see Table 5).

The ionised phenolic groups show absorption at 280 and 275 mm.

When reaction occurs between ®Xylok and silicone, no ionised phenolicgroups appear after addition of KOH.

TABLE 5 reaction time 2 hours 4 hours Catalyst Abs 280 nm Abs 275 nm Abs280 nm Abs 275 nm no catalyst 1.2 1.3 1.2 1.3 nonaloxy 1.2 1.2 1.3 1.3zirconate

The use of nonaloxy zirconate leads to results comparable to thoseobtained without a catalyst and both yields to products having asubstantial amount of remaining free phenolic groups.

What is claimed is:
 1. A polymeric composition for friction elements,comprising a co-polymer between (I) a resin containing phenolic groups,(II) an organopolysiloxane resin containing terminal silanol groups,whereby the reaction between the phenolic groups and the terminalsilanol groups is substantially complete, wherein tris neoalkalanatetitanate is used as a catalyst for the co-polymerisation reaction.
 2. Apolymeric composition according to claim 1, wherein the resin comprisingphenolic group comprises also terminal non aromatic alcoholic groups. 3.A polymeric composition according to claim 1, in which the resin (I)containing phenolic groups is of general formula (A) and may includemoieties of the general formula (A′):

wherein each of R₁ and R₂ is H, Akyl, or —CH₂OH.
 4. A polymericcomposition according to claim 3, wherein the resin comprising phenolicgroup is a phenyl aralkyl resin having a basic structure of alternatedmoieties A/A′.
 5. A polymeric composition according to claim 1, in whichthe resin containing phenolic groups is from 50 to 80% and theorganopolysiloxane resin containing terminal silanol groups is from 8 to25% by weight of the total starting mixture.
 6. A polymeric compositionaccording to claim 1, in which the organopolysiloxane resin containingterminal silanol groups is a diphenyl silicone or a hydroxy phenyl alkylsilicone resin.
 7. A polymeric composition according to claim 1, furtherincluding a reticulation agent which is hexamine.
 8. A polymericcomposition according to claim 1, in which the catalyst isneopentyl(diallyl)oxy trineodecanonyl titanate.
 9. A process ofpreparation of a polymeric composition, comprising the following steps:a) mixing and kneading together (I) a resin containing phenolic groupswith (II) a resin containing terminal silanol groups, in the presence ofa tris neoalkalanate titanate as a catalyst, for a period of timesufficient to substantially complete the reaction between the phenolicgroups and the terminal silanol groups, to yield a paste, b) coolingdown the paste until obtaining a solid, c) grinding the solid into apowder, d) adding a reticulation agent to said powder, e) curing theabove mixture, f) post-curing the product obtained under e).
 10. Aprocess according to claim 9, in which the tris neoalkalanate titanateis used in quantities of between 1 and 2% by weight of the totalmixture.
 11. A process according to claim 9 in which the curing step isdivided in a number of cycles permitting the degassing of the reactionmixture.
 12. A process according to claim 9 in which the starting resinsare in a form of powder with a particle size distribution of not morethan 400 μm.
 13. The polymeric composition of claim 1 as a binder forbrake pads.
 14. A polymeric composition according to claim 2, in whichthe resin (I) containing phenolic groups is of general formula (A) andmay include moieties of the general formula (A′):

wherein each of R₁ and R₂ is H, Akyl, or —CH₂OH.
 15. A polymericcomposition according to claim 14, wherein resin comprising phenolicgroup is a phenyl aralkyl resin having a basic structure of alternatedmoieties A/A′.
 16. A process according to claim 10 in which the curingstep is divided in a number of cycles permitting the degassing of thereaction mixture.
 17. A process according to claim 10 in which thestarting resins are in a form of powder with a particle sizedistribution of not more than 400 μm.
 18. The polymeric composition ofclaim 2 as a binder for brake pads.
 19. The polymeric composition ofclaim 3 as a binder for brake pads.
 20. The polymeric composition ofclaim 4 as a binder for brake pads.